Diamond is nature's best thermal conductor. Natural single crystal diamond has a thermal conductivity of 2000 W/(m.degree. C.) while the less expensive chemically vapor deposited (CVD) polycrystalline diamond has a thermal conductivity in the range from 500 W/(m.degree. C.) to 1600 W/(m.degree. C). The materials with the closest thermal conductivity to diamond are silver at 428 W/(m.degree. C.), copper at 396 W/(m.degree. C.), and aluminum at 237 W/(m.degree. C.). Silver, copper, and aluminum are electrically conductive while diamond is not (i.e., diamond is an insulator). Therefore, diamond does not require the use of an added insulating layer when used as a heat sink for an electronic device as does silver, copper, and aluminum. The thermal conductivity numbers for diamond, silver, copper, and aluminum indicate that for a given heat-sink specification, a diamond heat-sink would be smaller than a thermally-equivalent heat-sink made of silver, copper, or aluminum. Also, for a given heat-sink size, a diamond heat-sink would dissipate more heat than would an equally-sized heat-sink made of silver, copper, or aluminum.
Because of the foregoing reasons, diamond has received much attention as the substrate of choice for products that push the limit of either heat dissipation or size. The U.S. Department of Defense's Advanced Research Projects Agency (ARPA) is conducting research on diamond substrates as a packaging technology for its High-Performance Computing and Communication Initiative. ARPA envisions using diamond substrates to provide thermal management of its three dimensional (3D) supercomputer modules. A diamond substrate would provide the possibility of higher clock speed (i.e., higher performance), smaller size, and/or lower operating temperature (i.e., higher reliability).
To take full advantage of the thermal conductivity and the insulating characteristics of diamond, it would be beneficial to form conductive interconnections on the diamond substrate so that semiconductor chips/wafers may be placed directly onto the diamond substrate for ease of interconnection to another semiconductor chip/wafer and heat dissipation. Standard metals used for interconnect (i.e., gold, silver, and copper) do not adhere well to a diamond substrate. Untreated gold, silver, and copper tends to flake off of a diamond substrate. Numerous prior art patents such as U.S. Pat. No. 5,421,989 entitled "PROCESS FOR THE METALLIZATION OF NONCONDUCTIVE SUBSTRATES WITH ELIMINATION OF ELECTROLESS METALLIZATION"; U.S. Pat. No. 5,391,914 entitled "DIAMOND MULTILAYER MULTICHIP MODULE SUBSTRATE"; U.S. Pat. No. 5,382,758 entitled "DIAMOND SUBSTRATES HAVING METALLIZED VIAS"; 5,371,407 entitled "ELECTRONIC CIRCUIT WITH DIAMOND SUBSTRATE AND CONDUCTIVE VIAS"; U.S. Pat No. 5,334,306 entitled "METALLIZED PATHS ON DIAMOND SURFACES"; U.S. Pat. No. 5,262,042 entitled "SIMPLIFIED METHOD FOR DIRECT ELECTROPLATING OF DIELECTRIC SUBSTRATES"; and U.S. Pat. No. 5,190,796 entitled "IMPROVED METHOD OF APPLYING METAL COATINGS ON DIAMOND AND ARTICLES MADE THEREFROM," have all attempted to solve this problem by adding something to the interconnect metal such as a refractory metal, a noble metal, or graphite. All of the U.S. patents listed above are hereby incorporated by reference into the specification of the present invention.
When a refractory metal is used, it is typically sputtered onto a diamond substrate and heated to a temperature above 900.degree. C. A carbide of diamond results. Interconnect metal is then applied to the diamond-carbide. At 900.degree. C., the diamond substrate may be damaged by the hot atmosphere. Also, a metal interconnect layer applied to a refractory-metal-created diamond-carbide may blister or de-laminate.
Natural single crystal diamond is prohibitively expensive to use as a substrate. CVD polycrystalline diamond is expensive but not prohibitively so. Refractory metals are expensive and not very effective in achieving good adhesion between a metal interconnect layer and a diamond substrate. Therefore, there is a need for a reliable method of applying metal interconnect to a diamond substrate that is not prohibitively expensive.